Chemical milling in which strong acids or alkalis are used to etch away unneeded portions of a metal article is well known, especially in the aircraft industry where it is used to reduce the weight of aircraft parts. In the known process, a polymeric masking which resists the etching bath used is applied directly to the metal substrate, as by dipping. The applied mask is then scribed (cut through to base metal) using an appropriate template to allow desired portions of the applied mask to be peeled away to selectively expose those portions of the metal which it is desired to etch.
The character of the etching composition (etchant) will vary with the metal of the substrate. To illustrate this, an alkali bath is used to etch aluminum parts, and an acid bath is used to etch titanium parts. The rate at which the exposed metal is removed by the etchant will vary with its concentration and its temperature.
When the etching (chemical milling) process has been completed, the remaining mask is removed, and the etched part is appropriately rinsed, deoxidized if appropriate, and dried. In practice several dipping and drying steps are required to apply an appropriate mask, ready for scribing.
The normal masking composition used by most aircraft manufacturers today are rubber elastomers dissolved in organic solvents, such as toluene/xylene or perchloroethylene, the latter solvent being frequently employed because of its effectiveness. The coating systems which are in use are low solids content systems containing a high proportion of volatile organic solvent. The masking compositions are applied in two to three dipping operations in which the panel is dipped in the composition, excess material is dripped off, and the remainder is dried, usually in an oven. This process is then repeated until an appropriate mask thickness has been built up. A commonly used process is outlined below:
1. Apply first coat PA1 2. Bake at 100.degree. F. for 45 minutes PA1 3. Bake at 150.degree. F. for 45 minutes PA1 4. Cool PA1 5. Rotate the part "top for bottom" PA1 6. Apply second coat PA1 7. Bake at 100.degree. F. for 45 minutes PA1 8. Bake at 150.degree. F. for 45 minutes PA1 9. Remove large aluminum part PA1 10. Leave on conveyor all extrusion, true trim parts, parts shorter than eighteen inches. PA1 11. Apply third coat for parts left in Step 10 PA1 12. Bake at 100.degree. F. for 45 minutes PA1 13. Bake at 150.degree. F. for 45 minutes PA1 14. Remove parts left in Step 10 PA1 15. Rack masked titanium parts on separate rack PA1 16. Bake titanium at 225.degree. F. in separate oven PA1 17. Remove titanium parts
The solvents used in these systems are not exempt and must be considered as volatile organic content (VOC). Because of the low solids content of the organic solvent system used to apply the mask, the VOC of most of the systems is as high as 1200 grams per liter.
One method to reduce the VOC of the masking system is to use solvent recovery to reclaim most of the solvent emitted during the mask application process. Solvent recovery systems add complexity and expense.
Some aircraft manufacturers are currently using a solvent recovery system in conjunction with the use of perchloroethylene as the solvent in the masking solution. The entire coating system is enclosed, solvent being recovered from both the dipping and baking areas. The current efficiency of this system is 91%. The solvent is collected and used as a reducer in the masking solution without reprocessing.
The expense of building and operating such a system is obvious, leakage reduces its efficiency, and some perchloroethylene is retained in the mask film to be released in subsequent processing. Perchloroethylene presents a known carcinogenic risk which it is desired to avoid.